25 February 2017

a new S4285-like waveform (most likely a new Iraqi waveform)

The signal appears on-air as a PSK-8 modulation of a 1800Hz single tone, symbols rate is the quite usual 2400 Baud (Fig. 1). ACF is 120 msec, that makes 288 PSK-8 symbols per frame or a 864-bit length period (Fig. 2).

Fig. 1

Fig. 2

The structure of the frame resembles the well-known Stanag-4285 waveform: the initial 63 symbols preamble is followed by four data blocks, each block consisting of 51 unknown symbols (user data) and 7 known symbols (mini-probe); the last block is transmitted w/out the miniprobe (Fig. 3). As said, each frame is transmitted each 120 mesc.Looking at the constellation, the two pronounced points lead to think to a BPSK modulated preamble which is probably not scrambled (just as in Stanag-4285) and use Walsh modulation (Fig. 4).

Fig. 3

Fig. 4

The recording was sent me by KarapuZ and his friends, translators have identified the language as Iraqi (sometimes voice comms were copied in between the data exchanges) and are inclined to think to marine coast stations. Transmissions were copied on several USB frequencies such as 7997.0, 8228.0, 8250.0, 8458.0, 8613.0, 17380.0,...